1. Field of the Invention
The present invention relates to a start controller for an automatic transmission having a function of suppressing creep which causes fine speed advance of a vehicle without stepping on an acceleration pedal during the operation of an engine when a select lever is located at a forward advance position.
2. Description of the Prior Art
A typical vehicle including an automatic transmission permits a vehicle body to be brought into fine speed forward movement (hereinafter, referred simply to as creep.) when the automatic transmission is set to a forward advancement mode during the time an engine is operated but the vehicle is stopped. This is because that a first gear set to the automatic transmission in the stopped state has a high gear ratio, so that even slight driving force due to idle rotation of the engine is increased corresponding to the gear ratio of the first gear in a transmission mechanism and hence it is increased enough to rotate wheels.
To solve this, a method is presented in which a controller of an automatic transmission includes a creep suppression control function incorporated therein in which the automatic transmission is set to a second gear or higher during stoppage of a vehicle to automatically increase a speed change ratio whereby driving force of idle rotation of an engine does not get greater to rotate wheels through the transmission mechanism, eliminating the need of a driver to step on a brake pedal (otherwise, it will do for a driver to less step on).
An ordinary control function given to the controller of the automatic transmission is to control the speed change operation of the automatic transmission using allocation ranges of each gear that correspondes to throttle openings and vehicle speeds, to which function the first gear is in principle set when the vehicle is started.
Accordingly, in the controller of the automatic transmission having the creep suppression control function it is necessary to detect an operator's intention of starting the vehicle and to switch the control with the creep suppression function to the control with the ordinary control function.
Japanese Patent Application Laid-Open No. 62-184265 discloses an example of such a controller of an automatic transmission.
In this controller a vehicle speed and an engine revolution speed are detected to discriminate the operator's intention of continuation of stoppage and the operator's intention of a start, and sets a second gear to the automatic transmission for suppression of creep while interpreting the operator as having the intention of a start if the vehicle speed is less than a predetermined level, and returns the control to ordinary gear control when the engine revolution speed exceedes the predetermined level.
Viewed from the operator, the second gear is set to the automatic transmission to suppress the creep of a vehicle body when the operator has released his foot's hold from an acceleration pedal, while once the operator steps on the acceleration pedal to increase the engine revolution speed, the automatic transmission is immediately shifted down to a first gear and the vehicle body is powerfully accelerated, and thereafter it is changed to the second gear and to the third gear in response to the increase of the vehicle speed.
The controller of an automatic transmission disclosed in the Japanese Patent Application Laid-Open No. 62-184265 relatively succesfully functions for the creep suppression during the stoppage interval the operator has the intention of continuation of the stoppage, but it fails to control such a start operation that the automatic transmission is started with the second gear by discriminating specific operation conditions.
More specifically, when the operator steps on the acceleration pedal and hence the engine revolution speed is once raised, the control is automatically shifted to the first gear and substantially uniform start acceleration is achieved each time.
However, a second gear start is desired in several operation conditions. Namely, under the conditions where a vehicle is in a traffic jam with a short inter-vehicle distance and operations are repeated between stoppages and low speeds, a slow speed change and stable running are desired rather than a rapid response of a vehicle speed to operator's stepping on an acceleration pedal and powerful acceleration. In such a situation, an operator has the possibility of selecting a second gear start provided a manual shift transmission is installed on the vehicle.
Further, although in the case of a frozen road surface and a snow-covered road, a slow start and inertial running are ensured only by operator's setting of a particularly provided second gear start mode, in such a situation it is desired to discriminate the intention of an operator and automatically execute the second gear start for preparation of forgotten setting.
Furthermore, the engine revolution speed is increased against a load to rotate a torque converter so that it is increased, slightly delayed from the stepping on the acceleration pedal, more slowly than a change in throttle opening. It is therefore necessary to set predetermined level that is a threshold to transfer the operation to the ordinary control function to be lower in order to initiate acceleration in the ordinary control function without delaying so much from the stepping on the acceleration pedal.
Namely, if a start and acceleration are delayed even several seconds, although the operator steps on the acceleration pedal desiring a quick start and powerful acceleration, it is judged that operation properties are bad contrary to operator's expectation.
Idle speed of an engine is changed also owing to additional functions such as on and off operation of an air conditioner and an engine warming-up mode, so that when the predetermined revolution speed to transfer the operation to the ordinary control function are too low, the creep suppression control function is freely released even in the state where operator's foot is kept apart from the acceleration pedal to cause the possibility of any creep being produced irrespective of the intention of the operator.